Purchase the necessary ink mix at a local arts shop. At most arts stores, there will be pigmentation for purchase that you can use. You will need to purchase a base and then the purple pigmentation. The base is what you will add the color to. These will be sold in small tins or cups.
Add a tablespoon of the purple pigmentation to the base. Mix the ink up with a stirring utensil of some kind until the color is solid. This may take a few minutes of stirring. If the ink does not look like it is mixing, just keep stirring for a few minutes, as it may take time for it to combine.
Repeat this step until you have the shade of purple ink that your want. This step can be repeated as many times as needed, the shade of purple will just get darker with each new tablespoon of purple.